David, did you know there's a growing community out there who insists nuclear weapons are a hoax? They say that Nagasaki and Hiroshima were fire bombed, comparing photos to Dresden and other fire-bombed cities, so they say, the photos of then and now prove it, since they currently seem to be flourishing cities not affected by nuclear contamination. (Not making an argument here, just sharing info!)
It would explain the remarkable fact that no more weapons have been fired in anger, and both sides seem happy not test further. Even NK has not exploded a bomb openly. There have also been no accidental nuclear explosions, and no terrorist has ever managed to use one.
At one level, the physics of an atomic explosion seem simple - one neutron hits a fissile nucleus causing multiple neutrons to emerge to trigger more fission. However I`d bet you need secret information to calculate under what conditions you would get an explosion. Having seen how easily the science community can obfuscate the truth about a number of other subjects, it doesn`t seem impossible.